Metal casting cores



Aug. 4, 1964 N. s. LIRONES IETAL METAL CASTING CORES Filed April 6. 1961Illa-Ill "'1' "'6 INVENTORS Nlck G. L/r-mes y Theodore 0 M18 UnitedStates Patent 3,142,875 METAL CASTING CGRES Nick G. Lil-ones, NorthMuskegon, and Theodore Operhail, Whitehall, Mich assignors to Howe SoundCompany, New York, N.Y., a corporation of Delaware Filed Apr. 6, 1961,Ser. No. 191,201 4 Claims. (Cl. 22-196) This invention relates to newmetal casting cores useful in the casting of high melting point metalssuch as those contained in Group IV-B of the Periodic Table.Specifically, the invention relates to metal casting cores useful informing ceramic shell metal casting molds which may be employed in theproduction of castings of Group IVB metals.

In the propulsion of high speed aircraft, which use as part of theirpropulsion systems turbines and similar type devices, problems havearisen in connection with the materials to be used to produce componentsfor such systems. Such components must be capable of withstanding theextremes of high temperature and high stress which quite commonly occurin normal operation. To a lesser extent, similar problems arise in highpressure, high speed gas and steam turbines, and the production ofturbine components which have sufficient thermal properties to withstandthe extremes of heat and stress is thus made more critical.

In one approach to these problems, designers and manufacturers haveutilized such high melting point metals as titanium, zirconium andhafnium, and alloys thereof. However, the temperatures now encounteredin some of the newer aircraft and turbine equipment tend to cause thesematerials to fail. To overcome this barrier, a further approach hasinvolved modifications in equipment design, particularly of turbineblade construction.

It has become common practice to place a series of vents runningthroughout the interior surfaces of such components in an attempt toeffectuate air cooling which would keep these metal structures fromoverheating. When designing such air cooling type structures, it isdesirable to achieve maximum surface areas about which air can conductaway heat, yet, at the same time, not structurally weaken the component.

In order to produce air cooled structures of the type described above,particularly turbine components where high melting point metals are usedas materials of construction, it has frequently been the practice of theart to resort to manufacturing these products using a lost wax castingtechnique. In a preferred manufacturing operation, such parts are madeutilizing the special type lost wax casting process generally known as aceramic shell metal casting process.

It was soon found that the production of passages in turbine componentswas a difficult problem to solve using the ceramic shell molding processbecause of the difficulties experienced in making cores for the molds.The problem of providing cores for ceramic shell molds, when used tocast high melting point metals and alloys, includes the need for findingof a suitable core material which can withstand the rigors of thecasting process and for finding a suitable method for positioning andafiixing the cores within the mold. The problem is amplified, since, inthe case of producing cores for small castings, such as aircraft turbineblades or vanes, the cores used for producing openings in the componentsare quite frequently of small diameter, thus making their manufactureand use difficult due to the physical limitation of size.

Experiments conducted with various conventional core materials quicklyproved such materials to be woefully inadequate when used in producingcastings of high melting point metals and alloys by shell moldingprocesses,

ice

e.g., most ceramic cores tended to slump or crack when subjected to thehigh temperatures required in pouring these metals. When ceramics werechosen that had the proper thermal characteristics, it was found theirremoval from the finished part was accomplished with extreme difiiculty.Similarly, cores formed of low melting point metals such as copper,brass, iron and the like, failed completely since they melted during thepouring operation. High melting point metals and their alloys, whencapable of withstanding the pouring temperatures, tended to fuse withthe metal being poured, thus producing a contaminated casting.Non-metallic materials, such as graphite, tended to produce gaseousreaction products which formed air pockets throughout the castings thus,rendering them defective.

From the above discussion, it becomes evident that to be satisfactory asa core for casting high melting point metals in a ceramic shell metalcasting process, the particular core employed must possess the followingcharacteristics: (1) It must be easily formed; (2) will be of a typewhich will not gasify, slump or melt out at the temperatures used duringthe casting cycle; (3) be capable of easy removal after the casting hasbeen completed; and finally, (4) when used to produce the openings inair cooled parts, it should be capable of producing maximum surfaceareas in the finished part. These characteristics must be combined witha core capable of being accurately aflixed in a ceramic shell mold andsealed in relationship to such mold whereby it can be effectivelyutilized in producing finished metal parts.

It, therefore, becomes an object of this invention to produce a new typecore for use in casting high melting point metals and alloys, such asthe metals and alloys of Group lV-B, particularly adaptable for use in aceramic shell mold casting process.

Another object is to provide novel casting cores of the type describedabove which are capable of withstanding temperatures of such castingprocesses without deforming or being oxidized to a gaseous state andwhich are easily removed from the finished piece.

A specific object of this invention is to furnish a new type castingcore which is capable of producing particularly advantageous interiorsurfaces in finished parts such as high temperature turbine componentsand the like.

These and other objects of this invention will appear hereinafter andfor purposes of illustration, but not of limitation, specificembodiments of this invention are shown in the accompanying drawings inwhich FIGURE 1 is a perspective fragmentary view of a core of thisinvention;

FIGURE 2 shows in perspective a ceramic shell metal casting moldcontaining positioned therein a plurality of cores;

FIGURE 3 is an enlarged cross-sectional view taken along the line 33 ofFIGURE 2;

FIGURE 4 is a fragmentary cross-sectional view of a cast metal partcontaining a coating formed by the cores of the invention; and

FIGURE 5 is an enlarged cross-section taken along the line 4-4- ofFIGURE 4.

In accordance with the practice of this invention, it has been foundthat a suitable casting .core for use in producing high melting pointmetal castings by the ceramic shell metal casting process may beafforded by coating onto a core type material a uniform film of a highmelting point, difliculty oxidizable metal or alloy. The core typematerial makingup the interior portion of the core, may be composed ofany type solid substance which is capable of accepting a film of thehigh melting point difficultly oxidizable metal. Thus, for instance, thecore interior may be composed of either metals or non-metals,

although metals are preferred. Also, such non-metallic substances aswaxes and the like having been found suitable as the core base. In thecase of metals, any metal which has a melting point lower than themelting point of the high melting point, difficulty oxidizable metalouter coating may be used. Thus, such metals as copper, iron, steel,brass, admiralty metal, tin silver, bismuth, and the like may be used.In a preferred embodiment, it is desired to use as the inner portion ofthe core the metal molyb denum, which possesses special properties, whenused in the invention, that make it the most satisfactory under mostconditions.

The high melting point difiiculty oxidizable metal employed as the outersurface coating of the core is preferably selected from those metals andalloys thereof of this particular class which have a boiling point and/or melting point higher than the temperature of the metal poured intothe mold. Thus, with such materials as titanium, which are frequentlypoured at temperatures ranging from 28004 100 F., it is desirable to useas the outer coating such metals as chromium, platinum, tungsten,columbium, and tantalum, and alloys thereof. It will, of course, beunderstood that other metals possessing properties similar to thosedescribed above may also be employed in making the cores.

FIGURE 1 illustrates a typical core composed of an outer coat 14 of ahigh melting point, dih'icultly oxidizable metal which has been formedabout a suitable solid substance 12 which forms the interior of thecore. As previously indicated, in certain types of cores, the substance12 will be removed from the interior of the core, thus providing ahollow-type structure.

A ceramic shell mold 16 having positioned therein a plurality of cores10 adapted to form a series of voids in the interior of a finished metalcasting is shown in FIG- URE 2. The mold 16 is shown to be in the formof a turbine blade having a leading edge 30 and a trailing edge 32. Themold is formed of a plurality of ceramic coats in accordance with shellmolding pratcice, the coats being more clearly shown in FIGURE 3 bynumerals 18-27. The mold is provided with a pouring opening 28 intowhich molten metal is introduced at the time the casting is to be made.The cores 34 are located within the mold 16 and are positioned at theend portion 40 thereof. These cores may extend substantially into thesurface of the mold such as shown by cores 42 or the cores may beextended substantially beyond the end of the mold as is the case of thecore designated by the numeral 46. The opposite ends of the cores maylikewise pass through the other end of the mold, or locate within themold 16.

In order to accurately and rigidly affix the cores to the interiorsurfaces 38 of the mold, a series of holes was first formed into the waxor plastic pattern by using a suitable die designed with extensionswhich corresponded to the general dimensions of the core. Portions ofthe die corresponding to the cores which are to be employed extendbeyond the extremities of the pattern a distance sufiicient to provideopenings corresponding to the core positions. After formation orinjection of the wax or plastic into the die cavity, the die portionsare removed and the cores inserted in place.

As shown in FIGURE 3, the cores 10 will extend into the interior surface38 of the mold 16 throughout its entire thickness and beyond when usingthe above described expedient of a die to position the core within themold. The outer surface of cores 10 are fitted where they contact theinterior of the mold with expansion means 48 which are most suitablymade of the same ceramic as the composition of the mold, but may be ofother materials such as metal, wax, etc. Since, in most instances, thearea to be fitted by the core and the mold is extremely small, theexpansion means may be suitably formed by coating the end portion of thecore with a thin film of a substance which thus forms the expansionmeans. In the case of wax, the ends of the core would be dip coated withthe substance and then inserted into the wax or plastic pattern. Thecollar thus formed around the end of the core would then be allowed tobond to the interior of the mold.

Molybdenum which is a preferred substance from which the interior of thecore is fabricated is an extremely high melting point substance whichhas the unique property of forming gaseous products when employed underthe conditions of use in the invention. The interior molybdenum core isoxidized out after the casting has been made, leaving a thin shell ofthe high melting difiicultly oxidizable metal interior.

It has further been discovered, particularly in the case where chromiumwas used as the outer coating of the core, that a surface is producedbetween the metal core surface and the high melting point metal pouredaround the core which has special characteristics and unique properties.A microscopic inspection of the interior surface of the core showed itto appear as a roughened ridge-like surface. This surface 56 isillustrated in FIGURES 4 and 5. The chrome plating in the drawing isshown as being bonded to the inner surfaces 50 of the casting 51 at theinterface 52. The roughened surfaces are extremely desirable in turbinecomponents since they tend to produce maximum air turbulence, andincrease the cooling capacity of the passages.

When it is not desired to have the surfaces of the core become bonded tothe interior of the casting, suitable methods may be employed forremoving the thin coating. Such methods as acid etching, mechanicalburring, and the like may be used to remove the interior finish.

The cores may be formed using any suitable manufacturing technique.Thus, once the interior segment of the core has been formed either bycasting, extrusion, power metallurgical techniques or by the use ofadhesives, the outer film of the high melting point difiicultyoxidizable material is placed around the core by using known techniques,such as, for instance, electrical vacuum deposition, electroplating, dipcoating and the like. In the case of chromium or molybdenum, it wasfound that the expedient of chrome plating gave excellent results. Thechrome plate was applied, using conventional copper and nickel basecoatings prior to the actual deposition of the chrome film.

After the core has been formed, in some cases, it may be desirable toremove the interior of the core from the outer coating of the highmelting point, difiiculty oxidizable outer metal shell prior to actuallypouring the mold. When this is done, removal of the inner portion of thecore may be accomplished by using such removal technique as oxidation,chemical dissolution or physical removal methods such as drilling,suction and the like. When the interior of the core has been removed,what remains is essentially a hollow core which is composed of the highmelting point, difiiculty oxidizable metal. Such cores are invested withloose sand or the like for a back-up and used in ceramic shell molds aspreviously described.

The thickness of the coat, which comprises the outer layer of the coreis preferably about .005 inch but may be varied from between .0001 to.010 of an inch as long as the interior of the core has sufficientrigidity to maintain the shape and structure of the core throughout mostof the casting process. Where the core is removed prior to the castingoperation or is removed at an early stage of the casting process, thenit is desirable that the thickness of the exterior of the core be ofsufiicient magnitude to provide strength and rigidity, e.g., somewhatgreater than .005 inch. Chromium sleeves which may or may not be filledcan have inner diameters ranging from .030 to .050 inch and outerdiameters up to .080 inch.

The instant invention has been described with reference to shell moldingprocesses and a complete description of such processes may be found inTheodore Operhalls application Serial No. 708,628, entitled MetalCasting Process and Elements and Compositions Used in Same, filedJanuary 13, 1958, now Patent No. 2,961,751. It is understood, however,that the application of the inventive principles to other castingprocedures where the use of the inventive cores would be feasible iscontemplated.

It will be understood that various modifications in the above disclosedmetal casting cores may be provided without departing from the spirit ofthis invention, particularly as defined in the following claims.

We claim:

1. The process of forming castings of high melting point metals whereinsaid castings are provided with elongated internal passages, saidprocess comprising the steps of forming cores by providing a main bodyportion of a material adapted to be removed from the castings afterformation thereof, coating said main body portion with a metal selectedfrom the group consisting of chromium, platinum, tungsten, columbium,tantalum, and alloys thereof to provide a coating between .0001 and .01inch thick, imbedding said cores in an expendable pattern materialwhereby the ends of the cores extend outwardly of the pattern material,forming a ceramic shell mold around the pattern material whereby thesaid ends of the cores are at least partially imbedded in the shellmold, sealing the embedded portions of the core ends in the mold,removing the pattern material from said mold, and introducing said metalinto said mold to form said castlugs.

2. The process of claim 1, wherein the core is sealed into the mold byplacing a thin wax coating around a portion of the core embedded in themold.

3. The process of claim 1, wherein the core is sealed into the mold byplacing a ceramic collar around the portion of the core embedded in themold, said collar being composed of the ceramic used to form the mold.

4. The process of claim 1, wherein the core is sealed into the mold byplacing a metal collar around the core embedded in the mold, said collarhaving a coefficient of expansion similar to the coefiicient ofexpansion of the mold.

References Cited in the file of this patent UNITED STATES PATENTS1,912,889 Couse June 6, 1933 2,609,576 Rouse et al. Sept. 9, 19522,679,669 Kempe June 1, 1954 2,688,781 Fahlberg et al. Sept. 14, 19542,844,855 Gadd et al. July 29, 1958 2,880,486 Wallace Apr. 7, 19592,883,724 Hancock Apr. 28, 1959 FOREIGN PATENTS 826,340 Great BritainJan. 6, 1960 OTHER REFERENCES Modern Core Practices and Theories, byHarry W. Detert, published by American Foundrymens Association, 1942,pages 312, 313, 422-425.

1. THE PROCESS OF FORMING CASTINGS OF HIGH MELTING POINT METALS WHEREINSAID CASTINGS ARE PROVIDED WITH ELONGATED INTERNAL PASSAGES, SAIDPROCESS COMPRISING THE STEPS OF FORMING CORES BY PROVIDING A MAIN BODYPORTION OF A MATERIAL ADAPTED TO BE REMOVED FROM THE CASTINGS AFTERFORMATION THEREOF, COATING SAID MAIN BODY PORTION WITH A METAL SELECTEDFROM THE GROUP CONSISTING OF CHROMIUM, PLATINUM, TUNGSTEN, COLUMBIUM,TANTALUM, AND ALLOYS THEREOF TO PROVIDE A COATING BETWEEN .0001 AND .01INCH THICK, IMBEDDING SAID CORES IN AN EXPENDABLE PAT-